1. Field of the Invention
This invention relates generally to chronically implanted medical electrode leads and, in particular, to cardiac pacing leads with an electrode structure which minimizes chronic pacing thresholds and drain on the pacing pulse generator power source.
2. Description of the Prior Art
The safety, efficacy and longevity of an implanted pacemaker system depends (in part) on the performance of its pacing lead(s), the electronic circuits of the pacemaker pulse generator, the integrity of the pulse generator and the capacity and reliability of the pulse generator power source. These inter-related components of the pacemaker system optimally are matched in a fashion that accommodates ever increasing demands on the modes of operation and function of the system in conjunction with an overall reduction in its size, an increase in its longevity and an increased expectation in the reliability of the entire system. During the past thirty years, the technology of cardiac pacing has significantly advanced, with implantable pacemakers displaying an ever increasing variety of pacing modalities, substantially broadening the indications for pacemaker use. In conjunction with this advancement, there has been extensive research and development effort expended to optimize the performance of pacing leads and their reliability.
In the past ten years, substantial improvements in reliable stable chronic pacemaker stimulation and sensing thresholds have been achieved which in turn have allowed the development of smaller and longer-lived pacemakers that can be used with those leads with excellent safety margins and reliability. As new circuits are developed with lower "overhead" current drains, however, and as the circuits increase in complexity to allow for ever increasing pacemaker capabilities in their programmable functions, modes and memory, the longevity of the device depends increasingly more on the characteristics of the lead. In addition, implanters prefer that pacing lead bodies be made ever thinner, to occupy less space in the venous system, without diminishing or detracting from the mechanical strength and integrity of the lead body.
In the early days of cardiac pacing, very high geometric surface area electrodes were employed with bulky and short-lived pacemaker pulse generators. Early investigators including Dr. Victor Parsonnet advanced designs of pacing electrodes for achievement of low polarization and low thresholds while presenting a relatively small effective surface area for the delivery of a stimulating impulse in designs known as differential current density (DCD) of the type shown in U.S. Pat. No. 3,476,116. The DCD electrode (like all pacing electrodes of that time) suffered excessive chronic tissue inflammation and instability and was not pursued commercially.
Subsequent researchers, including Dr. Werner Irnich explored in considerable detail the electrode-tissue interface and sought to arrive at an optimum exposed electrode surface area for both stimulation thresholds and sensing. Dr. Irnich in "Considerations in Electrode Design For Permanent Pacing" published in Cardiac Pacing; Proceedings of the Fourth International Symposium of Cardiac Pacing (H. J. Thalen, Ed.) 1973, pages 268-274, argued that the field strength (E) required to stimulate varies as E=v.sub.r [1/r+d].sup.2 where v equals applied voltage (threshold, v), r equals electrode radius and d equals fibrous capsule thickness. He further argues that the mean value for d equals about 0.7 mm, regardless of electrode radius. Therefore, the smaller the electrode radius the lower threshold (assuming E is a constant) until r equals d. When r&lt;d, thresholds rise again. Dr. Irnich had concluded that the exposed hemispherical electrode at the tip of the lead should have a radius in the order of 0.7 to 1.0 mm which would result in an exposed surface area of 3-6 mm.sup.2. However, Dr. Irnich went on in his article to propose a somewhat different design employing wire hooks designed to penetrate the myocardium to hold the electrode in position. These active fixation wire hook electrodes never achieved popularity and were supplanted by passive fixation tined and active fixation screw-in endocardial pacing leads.
In a later paper, "Acute Voltage, Charge and Energy Thresholds as Functions of Electrode Size for Electrical Stimulation of the Canine Heart", by F. W. Lindemans and A. N. E. Zimmerman; Cardiovascular Research. Vol. XIII. No. 7, pp. 383-391, Jul., 1979, the author demonstrates that an electrode radius of about 0.5 mm is optimal in the acute situation. However, it was recognized that the benefits of a small electrode surface area would be lost when the fibrous capsule gets thicker than about 0.5 mm (as Irnich also states), and for that reason (and others stated in the article), electrodes of such small surface area could not be used chronically.
Dr. Seymour Furman had also studied the relationship of electrode size and efficiency of cardiac stimulation and presented a ball-tip/exposed spaced coil electrode and a small hemispheric electrode in his article entitled "Decreasing Electrode Size and Increasing Efficiency of Cardiac Stimulation" in Journal of Surgical Research, Volume 11 Number 3, Mar., 1971, pages 105-110. Dr. Furman concluded that the practical lower limit of electrode surface area was in the range of 8 mm.sup.2 observing that impedance increased as an inverse function of the surface area.
Electrodes of many shapes including cylindrical, ball-tip, corkscrew, ring tip and open cage or "bird cage" configurations were pursued with exposed electrode surface areas tending toward 8 mm.sup.2 in the mid 1970's.
More recently, various investigators have emphasized materials and their relationship to the considerations involved in optimizing electrode design. For example, the Medtronic U.S. Pat. No. 4,502,492 discloses a low polarization, low threshold electrode design of the early to mid 1980's which was commercialized as the "Target Tip.RTM." pacing leads in numerous models including Models 4011, 4012, 4511 and 4512. The tip electrode of the Target Tip.RTM. leads was generally hemispherical and provided with circular grooves. The electrode was fabricated of platinum, coated over its external surface with a plating of platinum black. The combination of the relatively low electrode surface area and platinum black contributed to state-of-the-art thresholds in that time period. Other manufacturers marketed porous platinum mesh (Cardiac Pacemakers, Inc.), totally porous sintered (Cordis Corporation), glassy and vitreous carbons (Siemens), and laser drilled metal (Telectronics Ppty. Ltd.) electrodes in that same time period.
A considerable breakthrough in the development of low threshold electrode technology occurred with the invention of the steroid eluting porous pacing electrode of Stokes U.S. Pat. No. 4,506,680 and related Medtronic U.S. Pat. Nos. 4,577,642, 4,606,118 and 4,711,281, all incorporated herein by reference. The electrode disclosed in the '680 patent was constructed of porous, sintered platinum or titanium, although carbon and ceramic compositions were mentioned. Within the electrode, a plug of silicone rubber impregnated with the sodium salt of dexamethasone phosphate or the water soluble forms of other glucocorticosteroids was placed in a chamber. The silicone rubber plug allowed the release of the steroid through the interstitial gaps in the porous sintered metal electrode to reach the electrode-tissue interface and prevent or reduce inflammation, irritability and subsequent excess fibrosis of the tissue adjacent to the electrode itself. The porous steroid eluting electrodes presented a source impedance substantially lower compared to similarly sized solid electrodes and presented significantly lower peak and chronic pacing thresholds than similarly sized solid or porous electrodes. Those two advantages of steroid eluting electrodes allowed the use of relatively small surface area electrodes of about 5.5 mm.sup.2 (CAPSURE.RTM. SP Model 5023, 5523 leads sold by Medtronic, Inc.) to raise the pacing impedance without sacrificing the ability to sense heart activity. The smaller electrode size permitted by the '680 patent invention resulted in higher current density during stimulation pulses, provided more efficient stimulation of the heart tissue with lower current drain from the implanted pacemaker power source. In addition, the localized nature of the drug treatment minimized the systemic assimilation of the drug and avoided undesirable side effects for the patient.
The 8 mm.sup.2 surface area CAPSURE.RTM. steroid eluting lead Models 4003, 4503, 4004, and 4504 sold by Medtronic, Inc. have enjoyed remarkable commercial success to the present time. However, many physicians are not taking full advantage of properties of the electrode to save battery current and, therefore, longevity attainable by programming pacemaker pulse voltage to a safety margin level above the thresholds afforded by these leads. The quest to provide even lower stimulation thresholds and improved sensing and otherwise increase the performance and reliability of the pacing leads continues. One objective is to achieve markedly lower stimulation thresholds and to convince the physicians to accept and program lower voltage stimulation pacing pulses.
The impedance of the lead as a whole is a function of the resistance of the lead conductor and the electrode tip as well as the effective impedance of the electrode-tissue interface. An inefficient way or means to raise impedance is to increase the resistance of the conductors. This wastes current as heat. It is preferable to decrease lead current drain with more efficient control of the electrode-tissue interface impedance. This can be done by reducing the geometric surface area of the cathode. However, it is commonly believed that small electrodes are inefficient at sensing natural depolarizations of the cardiac tissue. This is not necessarily true, however. The amplitude of the intrinsic cardiac depolarization signals (typically the ventricular QRS and/or atrial P-wave complexes) is essentially independent of electrode size, as measured on a high, megohm range input impedance oscilloscope. The problem is that the sense amplifiers of modern pulse generators have comparatively lower input impedance--typically about 35 k.OMEGA.. The impedance of the QRS or P-wave signal (or "source impedance") increases as the electrode surface area decreases. Thus, a 5 mm.sup.2 polished electrode will produce QRS or P-waves with about 5 k.OMEGA. source impedance. According to Kirchof's law, the attenuation of the signal in the generator's amplifier is 1/(1+Zin/Zs) where Zin is the input impedance of the amplifier and Zs is the source impedance of the signal to be sensed. Thus, a 5 k.OMEGA. signal into a 35 k.OMEGA. amplifier will have its amplitude reduced by 1/(1+35/5)=12.5%. In marginal cases, this may make the difference between being able to sense properly or not being able to sense. Therefore, it is important to keep the source impedance low, preferable to attenuate less than 5% of the cardiac signal, that is, Zs&lt;1800 .OMEGA., for a 35 k.OMEGA. amplifier.
Thus, there is a trade-off with geometric surface area of the cathode electrode between the demands for low current drain and adequate sensing. In addition, it is desirable to achieve relatively low polarization effects so that they do not distort the electrogram of evoked or intrinsic cardiac depolarizations or leave a postpulse potential of sufficient magnitude to be mistakenly sensed as a QRS or P-wave by the amplifier.